Magnetic structure



Dec. 6, 1960 L. J. ANDERSON MAGNETIC STRUCTURE Filed June 1, 1954 V if! rlllllllllllL I 'INVENTOR. Lr-sur- J. Amexsnnf A TTORNE 1' United States Patent MAGNETIC STRUCTURE Leslie J. Anderson, Moorestown, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed (lune 1, 19 54, Ser. No. 433,634

I 2 Claims. (Cl. 179--115.5)

This invention relates to magnetic circuits, and more particularly to magnetic structures associated with ribbon type microphones.

In constructing many types of ribbon microphones, it is often necessary to have proper spacing between magnetic members. A small variation in the dimensions of the magnets used often results in inefiiciency in the microphone. In the construction of such ribbon types of microphones, critical tolerances of the parts are, therefore, often involved.

In many types of magnetic structures associated with ribbons in ribbon microphones, the space between the magnets within the magnetic structure and the ribbon may be as little as .002 of an inch. It is important, therefore, that magnets not be so large as to lessen this air gap, since an interference with the free movement of the ribbon may result. It is also important that the air gap between the magnets and the ribbon not be too great, as would be the case if the magnets are too small, since the sensitivity of the microphone resulting from the structure would be greatly impaired. For example, if the space between the ribbon and the magnet is .005 of an inch instead of .002 of an inch, the sensitivity of the microphone would be reduced approximately 50 percent.

In accordance with the present invention, a small efficient magnetic circuit for a ribbon type microphone is provided, without involving critical tolerances of the parts. A pair of L or U-shaped members of substantially the same size is provided in a magnetic structure, each member having a magnet suitably attached thereto. These members provide the yoke pieces for the magnets and are mutually movable so as to provide accurate control of the air gap between the magnets. The assembly of the magnetic structure is accomplished by inserting a gauge of proper size in the air gap between the two magnets and overlapping portions of the U or L-shaped members. When the proper spacing has been provided, the overlapped L or U members are clamped in place. The members are then soldered, or otherwise suitably fixed to each other, thereby maintaining the desired air gap between the magnets within the magnetic structure.

It is an object of this invention to provide a small, eflicient magnetic circuit, which does not involve critical tolerances of the parts utilized in the circuit.

It is a further object of this invention to provide a small, efiicient magnetic structure for use in a ribbon type microphone.

It is a still further object of this invention to provide a method of manufacturing a magnetic structure, which does not involve critical tolerances of the parts included in the structure.

Other objects and advantages of the present invention will become apparent and suggest themselves to those skilled in the art to which the invention relates, from a reading of the following specification in conjunction with the accompanying drawings, in which:

Figure 1 is a view, in front elevation, of a magnetic icestructure utilizing a pair of L-shaped members in accordance with the present invention;

Figure l-a is a top plan view of the construction shown in Figure 1;

Figure 1-12 is a view, in side elevation, of the magnetic structure shown in Figure 1;

Figure 2 is a view, in front elevation, of a magnetic structure utilizing a pair of U-shaped members in accordance with the present invention; and

Figure 3 is an elevational view, partly in section, of a ribbon type microphone utilizing a pair of U-shaped members in accordance with the present invention.

Referring to Figures 1, 1a and 1-12, a pair of magnets 10 and 12 are soldered or otherwise suitably attached to a pair of L-shaped magnetic members 14 and 16, respectively. The L-shaped members provide the yoke assembly for the magnetic structure. The L-shaped members may be slightly tapered, as indicated to improve the efliciency of the magnetic circuit. The magnets have pole ends adapted to define an air gap and may also be tapered at these pole ends towards the air gap, as indicated, to improve the efiiciency of the magnetic circuit. A ribbon conductor may be suitably mounted and supported in the air gap between the magnets as shown in the structure of Figure 3. In carrying out the invention, a gauge of the proper size may be inserted in the air gap. The L-shaped members, being mutually slidable, are moved until the proper spacing of the air gap is attained. The members are then clamped in place and solder is applied to points 18 and 20, or other suitably holding means employed, to maintain the magnets in a desired spaced relationship.

Referring to Figure 2, the magnetic structure shown comprises a pair of magnets 22 and 24 suitably attached to a pair of U-shaped magnetic members 26 and 28. In assembly the structure, a gauge, shown here as two circular rods 30 and 31 are inserted in the air gap between the magnets. When the desired spacing is attained by moving the members 26 and 28 until the magnets come into physical relationship with the gauge, the members may be clamped in place. Heat and solder may'then be applied to points 32, 34, 36 and 38 to maintain the magnets in a fixed spaced relationship.

Referring now to Figure 3, a ribbon type microphone comprises a ribbon conductor 40 suitably disposed between a pair of magnets 42 and 44. The magnets are suitably at ached to a pair of U-shaped magnetic members 46 and 48. The ribbon is electrically connected to a transformer 50 through the leads 52, 54 and 56. The leads 52 and 56 are provided, in a known manner for balance. The electrical output of the transformer is coupled to a utilization circuit (not shown) through a cable 58. The U-shaped members are fixed to each other by means of solder 60 and 62, or by any other suitable means. The assembly of the magnetic structure of this ribbon type microphone is in accordance with the assembly previously described in connection with Figures l and 2. The entire microphone is suitably fixed to a base 64 and has a cover, or perforated screen, 66.

In assembly of the magnetic structure of Figure 3, the magnets 42 and 44 first are suitably attached to the intermediate portions 68 and 70, respectively, of the U-shaped members, or yoke pieces 46 and 48, respec tively. The magnets are disposed on the intermediate portions of the members between the top and bottom portions of the U-shaped members, with the top and bottom portions extending outwardly further from the intermediate portions than the magnets. The increased width of the top and bottom portions of the members permit an overlapping of the top and bottom portions of one member with top and bottom portions of the other member, while still maintaining an air gap between the magnets. When the proper size of the air gap is attained, by the insertion of a gauge between the magnets, for example, suitable means are employed to maintain the air gap between the magnets by physically attaching the members to each other.

It is seen that the present invention .provides a pair of mutually movable yoke pieces whereby accurate control of the air gap without critical tolerances on the parts is attained.

The magnetic structure involved in this invention is relatively simple and inexpensive to construct. The cost of constructing a ribbon microphone, where an accurate control of the air gap between the magnets .is necessary, is considerably reduced when the magnetic structure, embodying the present invention, is used.

What is claimed is:

1. A magnetic structure comprising .a pair of U-shaped elements, each of said elements having top, bottom and intermediate portions, -a pair of permanent magnets, means for attaching a diiterent one of said magnets to each of said intermediate portions of said .elements, said top and bottom portions of each of said elements extending in the same direction beyond said magnets, said elements being disposed with the top and bottom portions of one of said elements overlapping the top and bottom portions of the other of said elements and with said magnets opposed to each other to define an air gap therebetween, and means for securing said elements in fixed relationship with each other whereby the dimensions of said air gap between said magnets are fixed.

2. A ribbon type microphone having a magnetic structure comprising a pair of U-shaped members each having a pair of legs, a pair of permanent magnets each secured to a difierent one of said members between the legs thereof, said members being disposed with portions of one of said members overlapping portions of the other of said members and to provide an air gap between said magnets, means for securing said members in fixed relationship, and a ribbon conductor disposed within said air gap between said magnets.

References Cited in the file of this patent UNITED STATES PATENTS 1,748,993 Purdy Mar. 4, 1930 1,886,816 Jensen Nov. 8, 1932 1,941,476 Jensen Jan. 2, 1934 1,941,477 Jensen Jan. 2, 1934 1,995,025 Ellmore Mar. 19, 1935 2,020,211 Quam Nov. 5, 1935 2,020,212 Quam Nov. 5, 1935 2,046,515 Jensen July 7, 1936 2,233,886 Cowley et al. Mar. 4, 1941 2,307,205 Ewald et al. Jan. 5, 1943 2,501,615 Pugh Mar. 21, 1950 2,539,671 Olson Jan. 30, .1951 2,699,474 Olsonet al. Ian. 11, .1955 2,832,932 Baermann Apr. 29, 11958 FOREIGN PATENTS 155,495 Australia Dec. 13, 1951 168,557 Austria June 25, 1951 905,046 France Nov. 22, 1945 OTHER REFERENCES Magnetic Tape and Head Alignment Nomenclature by N. M. Haynes, published by Audio Engineering, June 1949, pages 22 and 23. 

